Making a turning on light bulb
There are several things unclear in the question, but I'll try to give a general answer:

if your light is just a Light object

you can keyframe its emission strength: 0 on the frame just before the switch goes on, your desired emission value on the frame when it is triggered. 

To keyframe almost any value button in the Blender Interface, just hover your mouse over it and press I

if your light is a mesh with a Cycles material *,

whose appearance when "off" is described by a single shader (e.g. a Diffuse) or a more complicated node tree:


open the Node editor, add a Mix Shader in the middle of your rightmost connection (the one that enters into "Material Output"). 

To the unconnected bottom-left socket of the Mix Shader add an Emission Shader node and give it your desired strength.
you can now keyframe the Fac: value of the Mix Shader: 0 means light off, something around 0.7 and 1 means light on (it depends on whether you want to keep some shading to the light-emitter material).


Alternatively you can use an Add Shader instead of the Mix Shader and keyframe the Emission strength instead of Fac:.

(* depending on how detailed is your light object, this could be the entire lightbulb or just the tungsten wire or whatever the "active" element is.)